Devices for causing reciprocal displacement along the longitudinal axis of a rotating device, such as the shaft of a pump, are generally well known in the art. These devices typically use the rotary motion of the rotating device, which may, for example, comprise a drive shaft in a compressor, to produce the reciprocating motion desired, which may, for example, be the motion required to drive a pumping device, such as a piston for compressing fluid.
One common way of transforming rotary motion into linear motion, particularly in pumps, is by the use of a swash or wobble plate, or similar mechanism, coupled to the shaft that employs a combination of both rotating and non-rotating parts to produce the reciprocating motion along the longitudinal axis of the shaft. The swash plate is typically coupled to one or more pumping devices, such as pistons, such that, when the plate pivots, the pistons reciprocate within piston channels of a cylinder block, alternately drawing fluid to be compressed into the channels of a cylinder block and subsequently compressing and discharging the fluid.
These pumps operate by employing a variety of different arrangements that utilize the rotational force of the drive shaft to make the swash plate pivot, such as an actuating assembly with a slanted surface underneath the swash plate, as disclosed in U.S. Pat. No. 6,439,857 to Koelzer and assigned to the assignee of the present application, an assembly of rotating and non-rotating plates, as disclosed in U.S. Pat. No. 5,626,463 to Kimura, and a rotatable cylinder block, as disclosed in U.S. Pat. No. 5,394,698 to Takagi. In these ways, the rotational force of the shaft is converted into linear motion of the pistons along the rotational axis of the shaft, enabling the pistons to alternately perform the functions of suction and compression, and thus, fluid is first drawn into a piston channel and is subsequently compressed and discharged from the piston channel.
One disadvantage of these types of pumps, however, is that they result in side loading—i.e., the pistons tend to tilt slightly within the piston channels as a result of the pivoting or wobbling motion of the plate. When side loading exists, the piston places greater stress on certain portions of the piston channel, resulting in greater wear and tear. Moreover, in some cases, severe side loading can result in a loss of an adequate seal between the piston and piston channel, compromising the integrity of the compression chamber.
In some pump designs, this undesired effect has been somewhat alleviated by the use of a wave shaped cam instead of a swash or wobble plate, such as those disclosed in U.S. Pat. No. 4,756,239 to Hattori et al., U.S. Pat. No. 5,655,953 to Murakami et al., and U.S. Pat. No. 6,481,393 to Drew. Unlike the pumps employing a swash or wobble plate described above, the rotating cam does not need to pivot. Instead, its wave shape creates a continually rising and falling surface as it rotates, thereby causing the pistons that are coupled to it to rise and fall as the cam rotates.
However, one problem with these devices is that, in order for the cam to drive the pistons in this manner, the cam must be coupled to the pistons, which must be coupled by some mechanism that permits relative motion between the cam and the piston stem. For example, in the aforementioned '239 and '393 patents, bearings are employed. In the aforementioned '953 patent, the pistons are coupled to the cam via semispherical shoes.
These arrangements result in a number of disadvantages. First, this relative motion between the piston and the cam still results in a certain degree of side loading, and thus, durability and compression may still be compromised. Moreover, they require multiple, complex parts, which are typically expensive and difficult to manufacture.
What is desired, therefore, is an axial displacement mechanism that minimizes the amount of side loading for a pumping device. What is further desired is an axial displacement mechanism for producing a pumping motion in a pump that is inexpensive to manufacture. What is also desired is an axial displacement mechanism for producing a pumping motion in a pump that is easy to assemble.